Transformer combo comparison in modular tube pre amp

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Holger

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This post is related to my modular tube pre amp system
https://groupdiy.com/index.php?topic=67518.0
I post it here because this is related to some specific transformer combos.

Recap: I've built a modular pre amp system based on Ian Thompson-Bell's TwinLineAmp.
The transfomer combos are

Module 1 Sowter 1948 1:10 IP, Sowter 1461 2k4:600 OP
Module 2 Sowter 1948 IP, custom wound ElectroMag 2k4:600 OP (there is a thread about this transfomer in this forum)
Module 3 OEP A187A15C 1:10 IP, EM 2k4:600 OP

I tried the OEP because I was annoyed about the long Sowter lead time (six weeks at that time, 12 weeks nowadays)
The OEPs are available within two days so I gave them a try.

I've recently discovered RoomEqWizard REW, so here are the measurements.
EDIT: measured with Antelope Orion 32+, amps set to line level input
First freq response: Sowter/Sowter
sowter%20ip%20sowter%20op.jpg

Sowter/Electro Mag
sowter%20ip%20EM%20op.jpg

OEP/Electro Mag
oep%20ip%20EM%20op.jpg

All
all.jpg

Conclusion freq wise: the Sowter/ElectoMag combos looks most promising, the OEP/Electromag is not so bad, but IIRC the
THD was much higher compared to the other combos (0.1% for Sowter IP compared to 0.5% for the OEP)Edit: wrong, see below
 
Looks like you have the same HF roll off problem with the OEP input transformer that I have observed. OEP are looking into this.

The 10Hz hump with the sowter:sowter combo  is probably the higher inductance of the output transformer resonating with the output cap

The EM output transformer has a higher insertion loss than the Sowter because its winding resistance is higher.

At what frequency and level did you measure distortion?

Cheers

Ian
 
These results show the problem with the Sowter output transformer at low frequencies and high levels. I clearly shows it saturating at 20Hz ant +10dBu. This is due to the mu metal in its core. The EM being made simply of M6 steel shows a much more modest rise in distortion at low frequencies and high levels, at the expense of slightly higher mid band distortion.

Cheers

Ian
 
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